The adaptive equalizer can effectively track a time-varying input signal in an unknown environment to correct in real time the spectral aberration of the received signal, so as to achieve the objectives of removing crosstalk among symbols in the time domain and realizing positive decision. FIG. 1 is a structural block diagram showing a prior art adaptive equalizer. As shown in FIG. 1, an analog input signal 101 passes through an analog-to-digital converter 103 controlled by a clock signal 102 to obtain a quantized digital signal 104 for subsequent equalizing process. A coefficient updating unit 107 obtains an updated equalizer coefficient in accordance with the input digital signal 104 and an output signal 106 of an adaptive equalizing unit 105, and inputs the coefficient to the adaptive equalizing unit 105 to make possible real-time tracking of changes in the system. It is usually required that frequency of the clock signal 102 be an integral multiple of the signal code rate, that is to say, a synchronous clock should be used. However, due to such severe system damages in the communications system as inter-code crosstalk, chromatic dispersion and polarization mode dispersion, there is great difficulty in extracting clock information.